Durban: Leading American environmentalists complained to Secretary of State Hillary Rodham Clinton on Thursday that her negotiators at UN climate talks risked portraying the US as an obstacle to fighting global warming because of its perceived foot-dragging on key issues.

Separately, European delegates and the head of the African bloc at the 192-party talks also denounced US positions at the talks, which are seeking ways to curb the ever-expanding emissions of heat-trapping greenhouse gases.

"Developed countries as a whole are not taking climate change seriously as a global issue," Seyni Nafo, of Mali, said. "Look at the US. We use and we welcome their leadership on democracy, on access to markets, on human rights issues. We would want to have the same leadership to tackle climate change, because for us in the developing world the biggest threat, the biggest enemy, is climate change."

Discontent directed at Washington came as the UN's top climate scientist, Rajendra Pachauri, warned the conference's 15,000 participants that global warming is leading to human dangers and soaring financial costs, but that containing carbon emissions will have a host of benefits.

Although he gave no explicit deadlines, the head of the Intergovernmental Panel on Climate Change implied that the world only has a few years before the Earth is irreversibly damaged by accumulations of carbon in the atmosphere.

The letter to Clinton signed by the chief executives of 16 major nonprofit groups also stressed the urgency of finding solutions to the world's emissions of carbon dioxide, mainly from burning fossil fuels for energy, industry and transportation.

"This is a critical meeting, and we are rapidly running out of time to avert the worst impacts of climate change," it said.

It reminded Clinton of President Barack Obama's Presidential campaign pledges to move the US back into the forefront of global cooperation on global warming.

"Three years later, America risks being viewed not as a global leader on climate change but as a major obstacle to progress," said the letter.

It was signed by the heads of the Environmental Defence Fund, the Sierra Club, the Natural Resources Defence Council and other major environmental lobby and activist groups.

Anger was directed at the US conditions for negotiating a deal that would legally bind all countries to limit their emissions, and for holding up discussion on how to raise USD 100 billion earmarked for poor countries to develop low-carbon economies and deal with the effects of global warming.

Instead of a binding agreement, the US has said it favours voluntary pledges by countries to do as much as they can to control emissions.